The box was originally designed for a truck a little heavier than what you have, but you would have room for it and there is nothing wrong with using it. I think the D ratio is a .86 overdrive. Its a lot of work to go about 9 miles an hour faster. Do you have all the shift linkage for it? The boxes are quite common, but the linkage is rather hard to find. You probably have the light 2 speed. They were available in 5.14/7.15, 5.83/8.10, 6.33/8.81 in GMC. I'm not sure what ratios Chev offered, but the rear ends interchange.
